Interest in high-efficiency energy storage devices has been growing steadily over the past several years and innovations in Li-ion battery technology have delivered improvements, in both power-density and energy-density, over Ni—Cd devices and the original industry workhorse-lead-acid batteries. Concurrently, interest in Electrical Double Layer Capacitors (EDLC)—also known as supercapacitors or ultracapacitors—has also spiked due to their potentially excellent power-density capabilities, and endurance for millions of cycles in conjunction with energy densities much higher than the traditional electrolytic capacitors. This has opened up EDLCs to new energy-storage applications in commercial environments for “energy-smoothing” and “transient high-load applications” (while traditional capacitors are mostly used as circuit components) and for short-term UPS (uninterrupted power supply) applications in which conventional batteries (low power) or conventional capacitors (low energy) are of little practical use.
Unlike batteries, supercapacitors do not undergo any Faradic electrochemical reactions and rely only on the rearrangement of ions in the electrolyte—near the electrode surface next to the electrodes—to form the electric double layer that stores the energy. Because this rearrangement process does not involve any charge transfer, it is much faster than the conventional Faradic reactions and the extent of the double layer formation (and hence the energy density) is theoretically proportional to the surface area of the electrodes. Thus, porous electrode materials with high surface areas are used in supercapacitor electrodes and carbon made from coconut shell charcoal (U.S. Pat. No. 6,589,904; Iwasaki et al.) has evolved as the material of choice due to its high surface area, wide availability and established manufacturing process. There are, however, some limitations faced by these electrodes that have limited the application of supercapacitors.
Energy density remains the key limiting factor in a wider acceptance of EDLCs in conventional energy storage applications; although other factors like the self-discharge rate and resistance also play important roles. While lab-scale devices with energy densities of 30 to 85 Wh/Kg have been announced (Liu, C., et al.; Nano Letters, 2010, 10(12):863-4868; Kang, Y. J., et al., Nanotechnology, 2012, 23(6):065401), commercially available devices today are typically rated at 5 Wh/Kg, much lower than Lithium ion batteries (100-150 Wh/kg) and even the conventional lead-acid batteries (40 Wh/Kg) (“Basic Research Needs for Electrical Energy Storage: Report of the Basic Energy Sciences Workshop on Electrical Energy Storage”; Apr. 2-4, 2007, Office of Basic Energy Sciences, DOE, July 2007).
All the commercially available supercapacitors today use activated carbon electrodes with surface areas of 1000 m2/g and above (Pandolfo, A. G. and A. F. Hollenkamp, A. F., J. Power Sources, 2006, 157:11-27; Burke, A. J., Power Sources, 2000, 91:37-50), and are made predominantly from coconut-shell charcoal, using powder-processing techniques to make electrodes from the activated carbon powder. Specific capacitance of these coconut-shell-charcoal-based devices is limited to ˜100 F/g in non-aqueous electrolytes—even with porous carbon electrode surface areas of 2000 m2/g and beyond. (See Barbieri, O. et al., Carbon, 2005, 43:1303-1310.) Several factors are believed to contribute to this behavior, including: 1) lack of control over porosity of the carbon electrodes; 2) impurities in the carbon; and 3) concentration of surface functional groups in the carbon material.
Coconut shell carbon (and by extension carbon from other natural sources) makes it difficult to control the inherent pore structure or to add other functional groups into the carbon to enhance performance by changing the surface chemistry. Also, concerns over removing impurities are high, since the starting materials already contain impurities from their natural sources.
Methods for producing nano-porous carbon for EDLC electrodes similar energy storage devices and are known. For example, Kuraray Chemical Corporation, Japan, which supplies most of the commercially available porous carbon for EDLC electrodes today, discloses a technique for making porous carbon from natural sources like coconut shell by acid activation at elevated temperature (U.S. Pat. No. 6,589,904; Iwasaki et al.). Downsides of this technique include concerns over impurity removal, limited sources of pore formation (activation process used after the carbon has been synthesized and cleaned), and no ability to introduce surface-functional groups into the carbon.
Techniques to make nano-porous carbon for EDLC applications from chemical reagents are known. Endo et al., disclose the use of low molecular weight, modified aromatic hydrocarbon resins (Endo, M. et al., Carbon, 2002, 40:2613-2626). Feaver et al. disclose the use of polymers of resorcinol and formaldehyde (U.S. Pat. No. 8,404,384). These techniques have not seen any commercial success due to relatively complex manufacturing methods and limited pore creation techniques.
Techniques for making nano-porous carbon for EDLC applications using templates which are subsequently removed to leave pores behind are known. Leis et al. disclose the use of titanium carbide (TiC) templates as starting materials to produce carbon particles with a dominating pore size of 7-8 Å inside the particles and over 8 Å in the surface layer of the particles (U.S. Pat. No. 7,803,345). Coowar discloses composite electrodes prepared from mesoporous nickel hydroxide, acetylene black and polytetrafluoroethylene with pore sizes between 1 and 50 nm grown from a liquid crystal templating medium (U.S. Published Application No. 2009/0170000, Jul. 2, 2009). These methods are relatively new and their commercial success remains unproven.